The invention relates to a sensor for measuring a magnetic field, comprising a substrate, a first magnetic element, a second magnetic element, a third magnetic element and a fourth magnetic element on the substrate in a bridge configuration, a first bridge portion wherein the first element and the second element are arranged in series and a second bridge portion wherein the third element and the fourth element are arranged in series being situated between a first contact and a second contact.
The invention also relates to a method of regulating a sensor for measuring a magnetic field, wherein a first magnetic element, a second magnetic element, a third magnetic element and a fourth magnetic element are situated on a substrate in a bridge configuration, a first bridge portion wherein the first element and the second element are arranged in series, and a second bridge portion wherein the third element and the fourth element are arranged in series being situated between a first contact and a second contact, and a first output contact being situated between the first element and the second element, and a second output contact being situated between the third element and the fourth element, a voltage being applied between the first contact and the second contact.
EP 0710850 shows a sensor for measuring a magnetic field. Magnetic sensors are used, inter alia, for reading data in a head for a hard disk or tape, or in the automobile industry for measuring angles and rotational speeds and to determine the position. Magnetic sensors have the advantage that they are comparatively insensitive to dust and enable measuring to take place in a contact-free manner. Sensors used for automotive applications must be resistant to high temperatures of approximately 200xc2x0 C.
In the known sensor, the resistance of the magnetic elements depends on the size and orientation of the magnetic field due to a magnetoresistance effect. The magnetic elements are arranged in a Wheatstone bridge configuration. The magnetic elements are spin valves. The spin valves comprise a pinned layer with a fixed orientation of the axis of magnetization and a layer with a free orientation of the axis of magnetization, which adopts the orientation of the magnetic field to be measured. The magnetoresistance value is determined, inter alia, by the angle between the axis of magnetization of the pinned layer and the freely rotatable axis of magnetization. In the Wheatstone bridge the axes of magnetization of the pinned layers in the bridge portions are oppositely directed. The difference in output voltage between the two bridge portions is converted to a differential amplitude voltage signal, which is a measure of the angle and the strength of the magnetic field. By virtue of said Wheatstone bridge configuration, the sensor is less sensitive to temperature than in the case of a single magnetoresistance element. The known sensor has become sensitive, however, to offset voltage and drift in offset voltage.
The offset voltage is defined as the output voltage of the bridge configuration in the absence of a magnetic field, and arises from differences in resistance values between the elements of the bridge. As a result of the fact that, in the known sensor, some magnetic elements of the bridge are locally heated in order to orient the axis of magnetization of the pinned layers, while others are not locally heated, differences in resistance values occur between the magnetoresistance elements.
The differences in resistance in the case of local heating are caused by interfacial mixing of the multilayers, as a result of which the magnetoresistance effect is reduced.
A drawback of the known sensor is that it cannot suitably be used to accurately measure magnetic fields in a large temperature range. The output voltage of the bridge circuit has an offset voltage at comparatively small magnetic fields. In addition, the known sensor is sensitive to fluctuations in temperature, as a result of which, inter alia, the offset voltage changes as a function of temperature, the so-termed drift in offset voltage.
It is an object of the invention to provide a sensor of the type described in the opening paragraph, which sensor can suitably be used to accurately measure magnetic fields in a large temperature range.
As regards the sensor in accordance with the invention, this object is achieved in that the first bridge portion includes an electric shunt resistor, which has a temperature coefficient and is electrically connected in parallel with the first magnetic element of the bridge.
The invention is based on the recognition that the offset voltage and the drift in the offset voltage are influenced predominantly by different parameters.
The electric resistance of a magnetoresistance element of the bridge corresponds approximately to R=R0(1+xcex1T), where R0 indicates the resistance at a temperature T=0xc2x0 C. and xcex1 is the temperature coefficient.
As the offset voltage is caused predominantly by a spread of the resistance values and drift in the offset voltage is caused predominantly by a spread of the temperature coefficients of the resistors, it is possible to separately compensate for the offset voltage and the drift in the offset voltage.
The drift in the offset voltage of the bridge configuration is compensated for in that the first bridge portion includes an electric shunt resistor that has a temperature coefficient and is electrically connected in parallel with first magnetic element of the bridge. If the overall resistance of the bridge changes as a result of a change in temperature, the shunt resistance changes also as a function of temperature, preferably in the opposite direction. The reduced drift in the offset voltage enables magnetic fields to be measured more accurately, particularly if fluctuations in temperature occur during operation.
Due to the compensating effect of the shunt resistor, the temperature coefficient of the shunt resistor preferably is of opposite sign to the temperature coefficient of the electric resistor of the first magnetic element. A large number of materials has a constant temperature coefficient over a large temperature range. Once the composition of the materials used for the magnetic elements is fixed, a material can be chosen for the shunt resistor having a temperature coefficient of opposite sign. The material of the shunt resistor is preferably provided in a separate deposition process. Subsequently, the material is structured by means of standard lithography and etching.
Advantageously, the absolute value of the temperature coefficient of the shunt resistor is larger than the absolute value of the temperature coefficient of the electric resistor of the first magnetic element. If so, the shunt resistor can suitably be used to compensate for comparatively large differences in drift of the offset voltage.
The shunt resistor may comprise at least one layer of a material, but it may alternatively comprise a plurality of layers such as, for example, in the case of a multilayer structure. To obtain a very accurate correction by means of the shunt resistor, the combination of a plurality of temperature coefficients of different layers of the shunt resistor can result in a better correction in a large temperature range.
Preferably, the shunt resistance is larger than the resistance value of the first magnetic element. As the shunt resistor is parallel-connected to the first magnetic element, a part of the current passes through the shunt resistor, so that the net magnetoresistance effect of the parallel connection is reduced. Favorably, the shunt resistance is chosen to be much larger than the resistance value of the magnetic element, for example a factor of 100. In this case, a reduction of the magnetoresistance effect by only maximally 1% takes place.
Advantageously, a first shunt resistor is arranged parallel to the first magnetic element and a second shunt resistor is arranged parallel to an element of the second bridge portion so as to reduce the drift in offset voltage in both bridge portions. It has been found that the drift in offset voltage is largest if the first element of the first bridge portion exhibits spread near the first contact, and the element of the second bridge portion exhibits spread near the second contact. As a second shunt resistor is situated, parallel to the element of the second bridge portion, near the first contact, positive as well as negative offset voltage drift can be compensated for.
Each magnetic element may comprise a number of paths of magnetic material which are mutually connected in series by electric conductors of, for example, a metal.
Advantageously, the resistance values of the electric conductors of each magnetic element are substantially the same to preclude that differences between resistance values of the bridge arise as a result of the electric conductors.
Although the resistivity of the electric conductors, such as Al or Cu, is generally much smaller than that of the material of the magnetic elements, the metal advantageously has a temperature coefficient that corresponds to the temperature coefficient of the magnetic material of the first magnetic element.
As the first bridge portion includes an electric trimmer resistor, which is connected in series with the first magnetic element and the shunt resistor, the offset voltage of the bridge is reduced. The trimmer resistor may comprise a circuit of resistors that are situated outside the bridge configuration. The resistors are preferably made of the same material as the magnetic elements. By means of a laser the resistors can be switched on or off. The trimmer resistor is regulated such that the offset voltage is substantially equal to zero.
Another object of the invention is to provide a method of manufacturing the sensor of the type described in the opening paragraph, by means of which the offset voltage and drift in the offset voltage can be regulated so as to become zero.
The object of the invention as regards the method is achieved, according to the invention, in that in the absence of a magnetic field, an output voltage between the first output contact and the second output contact of the bridge is reduced by means of a shunt resistor which is electrically connected in parallel with the first magnetic element in the first bridge portion.
The output voltage is preferably reduced at a temperature above room temperature. The differences in resistance values between the magnetic elements resulting from differences in temperature coefficient are generally larger at an increased temperature. The sensor can be regulated throughout the temperature range to which the sensor is exposed during operation.
Preferably, the shunt resistance can be adjusted. A suitable choice of materials enables the resistance value of the shunt resistor and the temperature coefficient to be accurately adjusted. The resistivity of the material, the thickness of the material and the geometry determine to a substantial degree the resistance value of the shunt resistor. The shunt resistor may comprise a circuit of a plurality of resistors.
Fine setting of the shunt resistance can be carried out by switching on or off several resistors of the circuit of the shunt resistor, using for example a laser.
After the drift in offset voltage has been reduced as much as possible by means of the shunt resistor, the output voltage of the bridge is reduced by means of an electrically adjustable trimmer resistor, which is electrically connected in series with the first magnetic element. The trimmer resistor is preferably also electrically connected in series with the shunt resistor. The offset voltage of the bridge is reduced at room temperature, said offset voltage being reduced substantially to zero.
These and other aspects of the sensor in accordance with the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.